1. Field of the Invention
This invention relates to a compactly constructed solid-state laser, particularly one that is efficient and of high power.
2. The Prior Art
Solid state lasers are known. See for example in the publication "Laser 89" (New Orleans), the Article entitled "CW-Frequency-Doubled Nd: YAG Laser with High Efficiency" by W. Rupp and P. Greve. The development and manufacture of high-power semiconductor laser diodes has made possible the design of compact, high-power and efficient solid-state lasers, wherein the laser diode is used as the pump light source of the solid-state laser. The spatial beam properties of the solid-state laser, e.g. divergence and beam profile are improved considerably over laser diodes. Laser diodes are used for the excitation of the solid-state laser, where in an optic-to-optic conversion efficiency .eta. of more than 40% can be attained. Since the emission wavelength of the laser diode can be tuned exactly to the absorption maximum of the solid-state crystal, the thermal loading of the laser crystal is reduced significantly in comparison to lamp pumps. For example, a lamp-pumped solid-state delivers as much as 40 W infrared light in CW operation or 14 W green light in multimode CW-operation, as indicated from the above-cited publication.
However, the total degree of efficiency of the above type of system is quite low. Several kilowatts of input power are required to create a few Watts of output power. Moreover, the system components, such as pump lamps, high voltage power supply, coolant system necessary for the pump lamps and laser crystal(s), are bulky. In addition, there is the disadvantage of a short life expectancy for the pump lamps.
The resonator of a diode-pumped solid-state laser normally has a linear, folded semi-monolithic, or ring-shaped design. One refers to a semi-monolithic design when a mirror, generally the input mirror, is formed directly on a laser rod crystal. The separate output mirror is provided with an optical coating, which has a transmission of a few percent for the fundamental wavelength.
In order to focus the semiconductor laser diode light in the laser crystal a lens system must be used. This lens system is optimized such that the overlapping of the pump light with the TEM.sub.00 -resonator mode in the laser crystal guarantees an efficient basic-mode operation. Thus the pump light should be imaged to the greatest extent possible within the resonator-mode space.
To double the fundamental frequency of the laser, up to now it has been necessary to employ an additional non-linear crystal, for example, a KTP crystal, in the resonator. A solid-state laser can be pumped by a diode laser either longitudinally or transversely. Transversely pumped systems generally exhibit a low degree of efficiency due to the unsuitable overlapping of TEM.sub.00 mode-and pump-volumes.
Most solid-state lasers use a single laser diode to end-pump the laser crystal. A few systems have been developed, which make it possible to couple more pump light in the laser crystal. In one such linearly-designed system, an Nd:YAG crystal is pumped from both sides by two GaA1As laser diodes, as has been shown in "Solid-State Laser Engineering" by W. Koechner, page 316, Springer Press, New York (1988). The crystal is located in the middle of the resonator between two mirrors. An outcouple mirror, external to the resonator, is required to outcouple the laser light which, however, makes it difficult to couple the pump light into the crystal efficiently.
A further possibility to pump a laser resonator longitudinally with several laser diodes, is yielded by polarization coupling of two laser diodes, as recommended in the Article "End-pumped Nd:BEL Laser Performance" by R. Scheps, J. Myers, E. K. Schimitschek, and D. F. Heller in "Optical Engineering 27" (1988), p. 830. In this case, however, the use of a beam-splitter cube leads to pump light losses of about 30%. As an alternative, the state of technology in fiber coupling recommends itself. Here the pump light of several laser diodes is coupled to the laser crystal longitudinally via fibers. However, this solution also exhibits considerable (light) losses.
The present invention overcomes the above shortcomings by providing a solid state laser system of improved efficiency and of reduced size, wherein multiple configurations and pulse operation, are possible and birefringent effects, through heat losses, are eliminated.